gofiber/fiber · error

only file paths and io.Writer are supported

Error message

only file paths and io.Writer are supported

What it means

Returned by Response.Save (client/response.go:230) when its argument is neither a string (file path) nor an io.Writer. Save's switch only handles those two destination kinds; any other type (int, struct, *os.File with nil, map) hits the default branch.

Solutions

1. Pass a string path to write to a file: resp.Save("/tmp/out.bin").
2. Pass an io.Writer (e.g. *os.File, *bytes.Buffer, *gzip.Writer) to stream the body: resp.Save(&buf).
3. For nil safety, check writers for nil before passing, especially *os.File.
4. If you have a custom Path type, call .String() to convert before Save.

Example fix

// before
var p int
err := resp.Save(&p) // ErrNotSupportSaveMethod

// after
f, _ := os.Create("body.bin")
defer f.Close()
err := resp.Save(f)

func isValidSaveDest(v any) bool {
    switch v.(type) {
    case string:
        return true
    case io.Writer:
        return true
    }
    return false
}

func isStringOrWriter(v any) bool {
    if _, ok := v.(string); ok { return true }
    if _, ok := v.(io.Writer); ok { return true }
    return false
}

if err := resp.Save(dest); err != nil {
    if errors.Is(err, fiber.ErrNotSupportSaveMethod) {
        // open a file or use bytes.Buffer and retry
    }
}

When it happens

Trigger: Calling resp.Save(someStruct), resp.Save(123), resp.Save(nil), or passing a channel/pointer-to-non-writer. Also passing an *os.File that is nil (it is not io.Writer in the nil-pointer case for the type switch).

Common situations: Refactoring Save to write to a buffer but passing *bytes.Buffer correctly works (it is io.Writer); passing the underlying byte slice instead of the buffer; passing a path object from a third-party package that isn't a plain string; nil receivers.

Related errors

• failed to write response body to file
• failed to write response body to writer
• the file should have a name
• client cannot be nil
• client: HTTPS to HTTP redirect blocked

AI-assisted analysis of gofiber/fiber@a105acad6c (2026-08-11). Data as JSON: /api/errors/dbd48676f4010e50. Report an issue: GitHub.

Appendix: source

Thrown at client/core.go:305

// - the channel is not closed
// - the channel is drained before returning it
// - the channel is not reused after returning it
func releaseErrChan(ch chan error) {
	errChanPool.Put(ch)
}

// newCore returns a new core object.
func newCore() *core {
	return &core{}
}

var (
	ErrTimeoutOrCancel      = errors.New("timeout or cancel")
	ErrURLFormat            = errors.New("the URL is incorrect")
	ErrNotSupportSchema     = errors.New("protocol not supported; only http or https are allowed")
	ErrFileNoName           = errors.New("the file should have a name")
	ErrBodyType             = errors.New("the body type should be []byte")
	ErrNotSupportSaveMethod = errors.New("only file paths and io.Writer are supported")
	ErrBodyTypeNotSupported = errors.New("the body type is not supported")
)

View on GitHub (pinned to a105acad6c)